Thanks for your help guys.

My hunch was right, I talked to the store yesterday and was told 1/4 of the way in from each side. Located the joists as well, the brackets go into a channel so any adjustments are easy. Today we paint.

Getting to 2,000 posts; one year at a time vp160/qs8/qs4/SVS 2000/m60/Monolith 3x200 amp